In the manufacture of electronic circuitry it is often necessary to protect part of the circuit panel from a chemical process while another part of the panel is exposed to that process. This protection is commonly provided in the printed circuit board industry by using a photoresist or screen printed resist when a pattern is to be transferred to a substrate or by using plastic backed pressure sensitive adhesive tape when image transfer is not required.
An example where protection is often required is thin film hybrid circuits where one side of the substrate contains a circuit pattern while the other side is a continuous metal coating that functions as an electrical ground plane. The thin film circuit is manufactured using an electroplating process wherein photoresist is applied to one side of the substrate and pressure sensitive adhesive tape is applied to the ground plane side.
In plating gold connector tabs on printed circuit boards, it is important to plate only the tabs themselves. To do this, in the circuitry art the tab end of the board is dipped into a gold electroplating bath. To provide definition of the tabs and to minimize gold waste, the portion of the board that may come into contact with the electroplating solution, but is not to be gold plated, is protected by plastic adhesive tape.
In the art, semiconductor fabrication requires that a resist be applied to the back side of a silicon wafer to protect it during photolithographic processes that occurs on the front side; this resist does not need to be photoimageable.
One of the important advances in printed circuit technology is the utilization of dry-film photoresists which are applied to a substrate by lamination with heat and pressure. This eliminates the need for the circuit manufacturer to coat and dry liquid photoresists, and the result is higher yield and better quality circuit production. The first such process is disclosed in U.S. Pat. No. 3,469,982 which utilizes a negative-acting photoresist. U.S. Pat. Nos. 3,526,504, 3,782,939 and 4,193,797 disclose the use of a positive-acting dry-film photoresist, and U.S. Pat. No. 4,247,616 discloses an improved dry-film positive-acting composition. However, these materials because of their photoimaging nature have a feature which is unnecessary and costly in many types of protective coating applications in the printed circuit industry.
Pressure-sensitive adhesive tapes have been used to mask circuitry that should be protected during electroplating or etching processes. While these materials themselves cost less than photosensitive resists, the labor costs involved in using tapes are significant. Labor is required to apply and trim the tape. Hand labor is required to pick the adhesive tape from the substrate after use. Following removal of the adhesive tape there is a residue that must be chemically removed from the surface of the board to insure proper long term electrical function.